Method for steering a vehicle with superimposed steering

ABSTRACT

A method of steering a vehicle with a superimposed steering system, wherein a steering angle input by the driver and an additional angle (additional steering angle) is determined and wherein the additional steering angle can override the input steering angle according to further quantities, in particular diving-dynamics quantities, by means of an electric motor, is characterized in that the method includes a steering angle control with a subordinated current or torque control of the electric motor.

TECHNICAL FIELD

The invention relates to a method of steering a vehicle with asuperimposed steering system, wherein a steering angle input by thedriver and an additional angle (additional steering angle) is determinedand wherein the additional steering angle can override the inputsteering angle according to further quantities, in particulardiving-dynamics quantities, by means of an electric motor.

BACKGROUND OF THE INVENTION

Up-to-date motor vehicles, in particular passenger vehicles, aregenerally equipped with hydraulic or electrohydraulic servo steeringsystems, wherein a steering wheel is compulsively coupled mechanicallywith the steerable vehicle wheels. The servo assistance is devised suchthat actuators, e.g. hydraulic cylinders, are arranged in themid-portion of the steering mechanism. A force generated by theactuators assists in the actuation of the steering mechanism in responseto the turning of the steering wheel. This reduces the force the driverhas to apply during the steering operation.

Superimposed steering systems are known in the art. They arecharacterized in that the steering angle input by the driver can beoverridden in case of need by another steering angle (additionalsteering angle) by means of an actuator. Usually electric actuators areemployed which act on an overriding drive and adjust the additionalsteering angle largely independently of the driver.

The additional steering angle is controlled by an electronic controllerand is e.g. used to increase the stability and agility of the vehicle.According to a prior art control concept, as described in DE 197 51 125A1, the steering components of the superimposed steering angle areproduced irrespective of each other.

BRIEF SUMMARY OF THE INVENTION

An object of the invention is to provide a method of steering a vehiclewith a superimposed steering that is safe and reliable in operation.

According to the invention, this object is achieved in that the methodincludes a steering angle control with a subordinated current or torquecontrol of the electric motor.

To this end, a nominal current or a nominal motor torque is produced bymeans of which the electric motor introduces an additional steeringangle into the steering system. Due to the angle superimposed on thesteering actuation, the desired steering angle and, hence, also theadditional steering angle is adjusted, which latter is additionallydemanded by other vehicle control systems, as the case may be.

It is arranged for in the invention that an actual steering angle valueand a nominal steering angle value is determined and, according to acomparison between the actual steering angle value and the nominalsteering angle value, a nominal current or a nominal motor torque isproduced by which the electric motor introduces the additional steeringangle into the steering system.

A favorable embodiment of the method of the invention includes that asteering request of the driver δ_(DRV) is determined on the basis of asteering wheel angle δ_(H) adjusted by the driver, wherein the driver'ssteering request δ_(DRV) is composed of the adjusted steering wheelangle δ_(H) and an invariably or variably predeterminable gear ratiofactor and the gear ratio factor is chosen corresponding to the currentdriving situation, in particular a detected longitudinal vehicle speedand/or a steering wheel turning angle, and that a nominal steering anglevalue δ_(nominal) is determined on the basis of the so calculatedsteering request of the driver and sent to the steering control.

According to another embodiment of the invention, the driver's steeringangle δ_(H) is determined and, in connection with a gear ratio factori_(L1), by which the driver's steering angle acts directly on thesteering gear, an additional steering angle δ_(M) is additivelysuperimposed thereon in connection with a second gear ratio i_(L2), anda superimposed steering angle δ_(L) is determined and sent as an actualvalue δ_(L,actual) to the steering control, with said superimposedsteering angle δ_(L) being determined according to the followingformula:δ_(L) =i _(L1)*δ_(H) +i _(L2)*δ_(M).

The invention provides that a driving dynamics control (ESP system)cooperates with the steering control and that an additional steeringangle Δδ responsive to driving dynamics is determined when the necessityof a stabilizing intervention is detected by driving dynamics control.

Preferably, the additional steering angle Δδ responsive to drivingdynamics that is produced on the basis of a correcting intervention of adriving dynamics controller is additively superimposed on the driver'ssteering request δ_(DRV).

The control of the superimposed steering is improved by this embodimentof the method of the invention in particular in highly dynamic drivingsituations. The term ‘highly dynamic driving situation’ refers to alldriving situations with a relatively quick change of the vehicledirection and/or the vehicle speed, which can cause instability of thevehicle or the desired vehicle movement. Driving situations in thefrontier of driving dynamics, such as skidding maneuvers, demand toomuch from many drivers regarding a suitable steering performance.

It is arranged for by the invention that based on the series steeringratio i_(L,series) and due to a boosting factor K1 responsive to asteering wheel angle and a boosting factor K2 responsive to the vehiclespeed, a resulting steering ratio I_(L,ESAS) which corresponds to theratio between the steered wheels δ_(V) and the driver's steering angleδ_(H) is determined according to the following formula:I _(L,ESAS)=δ_(V)/δ_(H) =i _(L,series)/(K 1*K 2)

According to the invention, an anticipatory control of the nominal speedof the motor ω_(M,nominal) is executed, which is determined from a motorspeed specification ω_(M,spec) and a motor speed preset value ω_(M,reg),and the motor speed preset value ω_(M,reg) is determined on the basis ofa comparison between a nominal steering angle value δ_(L,nominal) and adetermined actual steering angle value δ_(L,actual), and the motor speedspecification ω_(M,spec) is determined from the time derivative of thenominal steering angle value δ_(L,nominal) and the driver's steeringangle δ_(H) and a gear ratio factor i_(L2) by means of the followingformula:ω_(M,spec)=({dot over (δ)}_(L,nominal) −i _(L1){dot over (δ)}_(H))/i_(L2).

According to the invention, the control of the motor of the superimposedsteering is realized by a computer program which includes appropriateprogram steps for implementing the described method.

The above object is also achieved by a steering system for a vehicle,comprising a steering wheel arranged at a steering column, a steeringgear, a steering angle sensor arranged at the steering column, anoverriding motor that acts on the steering column by way of anoverriding gear, an electric steering control element, a sensor formeasuring the position of the steered wheels, and a steering controldevice, in which steering system the steering control device includes ameans for implementing the method of the invention describedhereinabove.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings,

FIG. 1 is a block diagram of the basic structure of the method of theinvention.

FIG. 2 is a block diagram of the structure of the method of theinvention.

FIG. 3 is a block diagram for determining a nominal value and an actualvalue as input quantities of the steering angle control according to theinvention.

FIG. 4 is a block diagram for determining a nominal value and an actualvalue as input quantities of the steering angle control according to theinvention.

FIG. 5 is a block diagram for determining a motor torque specificationfor the electric motor for adjusting the overriding angle according tothe invention.

FIG. 6 is a block diagram for determining a field weakening current anda nominal current for actuating the electric motor according to theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The basis structure of the method of the invention is represented inFIG. 1.

Based on the steering wheel angle δ_(H) 50 adjusted by the driver, thedriver's steering request δ_(DRV) 52 is calculated in the basic steeringfunction as a nominal steering angle value 53 δ_(L,nominal) (inputquantity) for the steering control circuit 54 by way of a variably orinvariably predeterminable gear ratio i_(L,ESAS) 51. In thisarrangement, the basic steering function generally comprises theselection of a steering ratio i_(L,ESAS) corresponding to the currentdriving situation, e.g. the detected longitudinal vehicle speed. Theactuator of the steering system is then driven corresponding to asteering angle δ_(L) 55 (output quantity of the control circuit 54).

Driving stability and agility of the vehicle can be enhanced by means ofadapting the position of the steered wheels, principally irrespective ofthe driver's request. To this end, an additional steering angle Δδ 56responsive to driving dynamics is additively superimposed 58 on thedriver's steering request δ_(DRV) 52 on the basis of a correctingintervention of a driving dynamics controller 57. The result is thenominal steering angle value δ_(L,nominal).

FIG. 2 shows the structure of the method. The driver's steering angleδ_(H) acts in the overriding gear as an input quantity 1 by way of amechanical gear 2 with a gear ratio factor i_(L1) directly on thesteering gear 3 (i_(L1)*δ_(H)) 19. The additional steering angle δ_(M)16 adjusted by a motor acts by way of a second gear 17 with a gear ratiofactor i_(L2) and is additively superimposed on the geared steeringangle of the driver:δ_(L) =i _(L1)*δ_(H) +i _(L2)*δ_(M).

The steering gear 3 generates as an output quantity a resulting steeringangle δ_(V) that acts upon the vehicle.

The driving dynamics of the vehicle 5, especially the yaw torque aboutthe vertical axis of the vehicle 5, and the transverse acceleration aredetermined. The driving-dynamics quantities 7 and the driver's steeringangle δ_(H) 8 are sent as input quantities to a driving dynamicscontroller 6. Driving-dynamics-related steering interventions in thecapacity of an additional steering angle Δδ 9 are sent as an inputquantity to a steering controller 10 by means of the driving dynamicscontroller 6. Likewise, the driver's steering angle δ_(H) 11 and a valuefor the present vehicle speed 12, in particular the vehicle referencespeed from the driving dynamics controller 6 or an ABS controller, issent as an input quantity to the steering controller 10. Said steeringcontroller 10 drives the actuator 14 of the overriding steering function15.

The actuator, in particular an electric motor 14, produces an additionalsteering angle δ_(M), which acts by way of a gear 17 with a gear ratiofactor i_(L2) on the steering gear 3 (i_(L2)*δ_(M)) 18. Gear 2 and gear17 are illustrated herein as two individual ‘gears’ only forrepresentation purposes. However, the two gear ratios of gears 2 and 17are preferably realized by way of one single gear unit, in particular aplanetary gear.

As can be taken from FIG. 1 already, the additional steering angle Δδwhich shall be considered as an external intervention of the drivingdynamics controller 6 is additively superimposed at 58 on the nominalsteering angle δ_(DRV) of the basic steering function. The nominalsteering angle value δ_(L,nominal) resulting from this addition is sentto the control of the superimposed steering.

A sum steering angle δ_(L) 21 is the result of the additivesuperposition of driver's steering angle and superimposed steering anglegenerated by the actuator, from which sum steering angle a resultingsteering angle δ_(V) is produced by the steering gear 3 as a resultingoutput quantity and acts on the vehicle corresponding to the desiredlearning function.

The resulting steering angle δ_(L) 21 is furnished to the steeringcontroller 10 as an input quantity at 22, just as the additionalsteering angle δ_(M) 23. The resulting steering angle δ_(L) is also sent26 as an input quantity to the driving dynamics controller 6. Signals ormeasured quantities of the actuator means, the electric motor 14, arealso sent to the steering controller 10 at 24.

FIG. 3 shows the determination of the nominal steering angle valueδ_(L,nominal) and, if needed, a motor speed specification ω_(M,spec) 44in a nominal value producing means 30 and the determination of theactual value δ_(L,actual) in an actual value producing means 31, saidvalues being used as input quantities 32, 33 of the steering controller34 under consideration. A motor torque M_(mot,nominal) 35 to be adjustedor a torque-producing motor current l_(q,nominal) is produced fromoutput quantities. These quantities are associated with the electricmotor, exactly as a commutation of the motor (in the case of anelectronic commutation).

In this arrangement, the control quantity of the steering controller 34is the steering angle δ_(L), which is either directly measured and sent36 to the actual value producing means 31, or which can be calculated inthe actual value producing means 31 by means of the motor angle δ_(M) 37and the driver's steering angle δ_(H) 38 in consideration of the gearratio of the overriding gear. The motor speed ω_(M,actual) 40 which canbe calculated from the measured motor angle by differentiation is usedas internal control quantity.

The driver's steering angle δ_(H) 41 and the additional steering angleΔδ 42 and the vehicle speed V_(VEH) 43 are also sent to the nominalvalue producing means.

FIG. 4 shows the determination of the nominal steering angleδ_(L,nominal) 32 in greater detail.

The resulting steering ratio i_(L,ESAS) 60 corresponds to the ratiobetween the angle of the steered wheels (wheel turning angle) δ_(V) andthe driver's steering angle δ_(H). It results from two boosting factorsK1 61 and K2 62 which are multiplicatively combined with the seriessteering gear ratio i_(L,series) by the following formula:i _(L,ESAS)=δ_(V)/δ_(H) =i _(L,series)/(K 1*K 2).

The boosting factors represent a component K1 responsive to the steeringwheel angle 63 and a component K2 responsive to the vehicle speed 64.They can be chosen freely according to aspects related to drivingdynamics or specifications by the driver. To calculate the nominalsteering angle value δ_(L,nominal) and the motor speed specificationω_(M,spec) 66, the additional steering angle Δδ 67 is also taken intoconsideration, and a corrected additional steering angle Δδ_(IPO) 69 issuperimposed at 71 on the driver's request δ_(nominal,DRV) 70 after aninterpolation and limitation of rise 68.

The motor speed specification ω_(M,spec) 66 is calculated from the timederivative of the nominal steering angle value δ_(L,nominal) and thesteering angle of the driver δ_(H) by the following formula 72:ω_(M,spec)=({dot over (δ)}_(L,nominal) −i _(L1)δ_(H))/i _(L2).

FIG. 5 shows the steering angle control in greater detail. Said controlis a cascade control in its basic structure. An anticipatory control ofthe nominal speed of the motor is executed to enhance the dynamics ofthe control circuit. The nominal speed ω_(M,nominal) is produced 83 fromthe motor speed specification ω_(M,spec) 81 and the motor speed presetvalue ω_(M,reg) 93 being determined as an output quantity of the anglecontroller based on the comparison between the nominal steering anglevalue δ_(L,nominal) and the actual steering angle value δ_(L,actual)determined. To prevent impairment of the steering comfort by theanticipatory control especially during slow steering movements, theanticipatory control value is weighted depending on the desired motorspeed at 83, 84.

The nominal motor torque M_(mot,nominal) 86 or a torque-producingnominal motor current I_(q,nominal) 87 by which the motor shall bedriven, is produced from the nominal speed ω_(M,nominal) 80 and thecomparison with the actual motor speed ω_(M,actual) 88 determined by wayof a motor speed controller 85.

A higher motor speed than available may be required in certain cases ofoperation. In this case, a demand-responsive brief increase of the motorspeed without reduction of the available motor torque can be reached byusing a field weakening. A brief increase of the current consumption isrelated thereto. In particular the existence of a very direct steeringratio and a high nominal speed on the part of the driver or the drivingdynamics control system is considered as a case of need. The resultingcontroller structure represents an extension of the structure shown inFIG. 5 and is illustrated in FIG. 6. Therefore, all steps and elementscorresponding to FIG. 5 have been assigned equal reference numerals inFIG. 6 and will not be explained in detail in the following.

A decision about the use of the field weakening and the magnitude of thefield weakening current is taken 104 based on the present actualcondition of the steering system, that means the prevailing actual motorspeed 107 _(M,actual) 100 and the prevailing steering angle valueδ_(L,actual) 101 as well as the desired nominal condition, i.e. themotor speed specification ω_(M,spec) 102 and the nominal steering anglevalue δ_(L,nominal) 103 and the boosting factors of the steering ratio106. In case field weakening of the motor is not necessary, theresulting field weakening current I_(dnominal) 105 is zero, i.e. 0 A.The torque control of the electronically commutated motor is thenrequired to control the field-weakening current value Id in addition tothe torque-producing current Iq 87.

1-12. (canceled)
 13. Method of steering a vehicle with a superimposedsteering system, wherein a steering angle input by the driver and anadditional angle (additional steering angle) is determined and whereinthe additional steering angle can override the input steering angleaccording to further quantities, by means of an electric motor, whereinthe method includes a steering angle control with a subordinated currentor torque control of the electric motor.
 14. Method as claimed in claim13, wherein an actual steering angle value and a nominal steering anglevalue is determined and, according to a comparison between the actualsteering angle value and the nominal steering angle value, a nominalcurrent or a nominal motor torque is produced by which the electricmotor introduces the additional steering angle into the steering system.15. Method as claimed in claim 13, wherein a steering request of thedriver δ_(DRV) 52 is determined on the basis of a steering wheel angleδ_(H) 50 adjusted by the driver, and wherein the driver's steeringrequest δ_(DRV) 52 is composed of the adjusted steering wheel angleδ_(H) 50 and an invariably or variable predeterminable gear ratio factorand the gear ratio factor is chosen corresponding to the current drivingsituation, and wherein a nominal steering angle value 53 δ_(nominal) isdetermined on the basis of the so calculated steering request of thedriver and sent to the steering control.
 16. Method as claimed in claim13, wherein the driver's steering angle δ_(H) is determined and, inconnection with a gear ratio factor i_(L1) by which the driver'ssteering angle acts directly on the steering gear, an additionalsteering angle δ_(M) is additively superimposed thereon in connectionwith a second gear ratio i_(L2), and wherein a superimposed steeringangle δ_(L) is determined and sent as an actual value δ_(L,actual) tothe steering control, with said superimposed steering angle δ_(L) beingdetermined according to the following formula:δ_(L) =i _(L1*)δ_(H) +i _(L2)*δ_(M).
 17. Method as claimed in claim 13,wherein a driving dynamics control (ESP system) cooperates with thesteering control, and wherein an additional steering angle Δδ 56responsive to driving dynamics is determined when the necessity of astabilizing intervention is detected by driving dynamics control. 18.Method as claimed in claim 13, the method further comprising a drivingdynamics control (ESP system) cooperates with the steering control, andan additional steering angle Δδ 56 responsive to driving dynamics isdetermined when the necessity of a stabilizing intervention is detectedby driving dynamics control, wherein the additional steering angle Δδ 56responsive to driving dynamics that is produced on the basis of acorrecting intervention of a driving dynamics controller 57 isadditively superimposed on the driver's steering request δ_(DRV)
 52. 19.Method as claimed in claim 13, wherein the electric motor isadditionally actuated by means of a field weakening current according tofurther quantities, with a view to increasing the motor speed withoutreduction of the available motor torque.
 20. Method as claimed in claim13, the method further comprising the electric motor is additionallyactuated by means of a field weakening current according to furtherquantities, with a view to increasing the motor speed without reductionof the available motor torque, wherein the electric motor isadditionally actuated by means of a field weakening current when a verydirect steering ratio and/or a high nominal speed is desired orrequired.
 21. Method as claimed in claim 13, wherein based on the seriessteering ratio i_(L,series) and due to a boosting factor K1 responsiveto a steering wheel angle and a boosting factor K2 responsive to thevehicle speed, a resulting steering ratio I_(L,ESAS) which correspondsto the ratio between the steered wheels δ_(V) and the driver's steeringangle δ_(H) is determined according to the following formula:i _(L,ESAS)=δ_(V)/δ_(H) =i _(L,series)/(K 1*K 2).
 22. Method as claimedin claim 13, wherein an anticipatory control of the nominal speed of themotor ω_(M,nominal) is executed, which is determined from a motor speedspecification ω_(M,Spec) and a motor speed preset value ω_(M,reg), andthe motor speed preset value 107 _(M,reg) is determined on the basis ofa comparison between a nominal steering angle value δ_(L,nominal) and adetermined actual steering angle value δ_(L,actual), and the motor speedspecification coM,spec is determined from the time derivative of thenominal steering angle value δ_(L,nominal) and the driver's steeringangle δ_(H) and a gear ratio factor i_(L2) by means of the followingformula:ω_(M,spec)=(δ_(L,nominal) −i _(L1)δ_(H))/i _(L2).
 23. Computer program,which is suitable for implementing a method of steering a vehicle with asuperimposed steering system, wherein a steering angle input by thedriver and an additional angle (additional steering angle) is determinedand wherein the additional steering angle can override the inputsteering angle according to further quantities, by means of an electricmotor, wherein the method includes a steering angle control with asubordinated current or torque control of the electric motor. 24.Steering system for a vehicle, comprising a steering wheel arranged at asteering column, a steering gear, a steering angle sensor arranged atthe steering column, an overriding motor that acts on the steeringcolumn by way of an overriding gear, an electric steering actuator, asensor for measuring the position of the steering wheels, and a steeringcontrol device, wherein the steering control device includes a means forimplementing the method as claimed in any one of the preceding claims.